Standard conforming

Description

The ctime() function converts the time pointed to by clock, representing the
time in seconds since the Epoch (00:00:00 UTC, January 1, 1970), to
local time in the form of a 26-character string, as shown below.
Time zone and daylight savings corrections are made before string generation. The fields
are in constant width:

Fri Sep 13 00:00:00 1986\n\0

The ctime() function is equivalent to:

asctime(localtime(clock))

The ctime(), asctime(), gmtime(), and localtime() functions return values in one of
two thread-specific data objects: a broken-down time structure and an array of
char. Execution of any of the functions can overwrite the information returned in
either of these objects by any of the other functions executed by
the same thread.

The ctime_r() function has the same functionality as ctime() except that the
caller must supply a buffer buf with length buflen to store the
result; buf must be at least 26 bytes. The standard-conforming ctime_r() function does
not take a buflen parameter.

The localtime() and gmtime() functions return pointers to tm structures (see below).
The localtime() function corrects for the main time zone and possible alternate
(“daylight savings”) time zone; the gmtime() function converts directly to Coordinated Universal
Time (UTC), which is what the UNIX system uses internally.

The localtime_r() and gmtime_r() functions have the same functionality as localtime() and
gmtime() respectively, except that the caller must supply a buffer res to
store the result.

The asctime() function converts a tm structure to a 26-character string, as
shown in the previous example, and returns a pointer to the string.

The asctime_r() function has the same functionality as asctime() except that the
caller must supply a buffer buf with length buflen for the result
to be stored. The buf argument must be at least 26 bytes.
The standard-conforming asctime_r() function does not take a buflen parameter. The
asctime_r() function returns a pointer to buf upon success. In case
of failure, NULL is returned and errno is set.

Declarations of all the functions and externals, and the tm structure, are
in the <time.h> header. The members of the tm structure are:

The value of tm_isdst is positive if daylight savings time is in
effect, zero if daylight savings time is not in effect, and negative
if the information is not available. Previously, the value of tm_isdst was
defined as non-zero if daylight savings was in effect.

The external time_t variable altzone contains the difference, in seconds, between Coordinated
Universal Time and the alternate time zone. The external variable timezone contains
the difference, in seconds, between UTC and local standard time. The external variable
daylight indicates whether time should reflect daylight savings time. Both timezone and
altzone default to 0 (UTC). The external variable daylight is non-zero if
an alternate time zone exists. The time zone names are contained in the
external variable tzname, which by default is set to:

char *tzname[2] = { "GMT", " " };

These functions know about the peculiarities of this conversion for various time
periods for the U.S. (specifically, the years 1974, 1975, and 1987). They
start handling the new daylight savings time starting with the first Sunday
in April, 1987.

The tzset() function uses the contents of the environment variable TZ to
override the value of the different external variables. It is called by
asctime() and can also be called by the user. If TZ is
not specified or has an invalid setting, tzset() uses GMT0. See environ(5) for
a description of the TZ environment variable.

Starting and ending times are relative to the current local time zone.
If the alternate time zone start and end dates and the time
are not provided, the days for the United States that year will
be used and the time will be 2 AM. If the start
and end dates are provided but the time is not provided, the time
will be 2 AM. The effects of tzset() change the values of
the external variables timezone, altzone, daylight, and tzname.

Note that in most installations, TZ is set to the correct value
by default when the user logs on, using the local /etc/default/init file
(see TIMEZONE(4)).

Return Values

Upon successful completion, the gmtime() and localtime() functions return a pointer to
a struct tm. If an error is detected, gmtime() and localtime() return a
null pointer.

Upon successful completion, the gmtime_r() and localtime_r() functions return the address of
the structure pointed to by the res argument. If an error is
detected, gmtime_r() and localtime_r() return a null pointer and set errno to indicate
the error.

Errors

The ctime_r() and asctime_r() functions will fail if:

ERANGE

The length of the buffer supplied by the caller is not large enough to store the result.

The gmtime(), gmtime_r(), localtime(), and localtime_r() functions will fail if:

EOVERFLOW

The result cannot be represented.

Usage

These functions do not support localized date and time formats. The strftime(3C)
function can be used when localization is required.

The localtime(), localtime_r(), gmtime(), gmtime_r(), ctime(), and ctime_r() functions assume Gregorian dates.
Times before the adoption of the Gregorian calendar will not match historial records.

Examples

Example 1 Examples of the tzset() function.

The tzset() function scans the contents of the environment variable and assigns
the different fields to the respective variable. For example, the most complete
setting for New Jersey in 1986 could be:

EST5EDT4,116/2:00:00,298/2:00:00

or simply

EST5EDT

An example of a southern hemisphere setting such as the Cook Islands
could be

KDT9:30KST10:00,63/5:00,302/20:00

In the longer version of the New Jersey example of TZ, tzname[0]
is EST, timezone is set to 5*60*60, tzname[1] is EDT, altzone is
set to 4*60*60, the starting date of the alternate time zone is the
117th day at 2 AM, the ending date of the alternate time
zone is the 299th day at 2 AM (using zero-based Julian days),
and daylight is set positive. Starting and ending times are relative to
the current local time zone. If the alternate time zone start and end
dates and the time are not provided, the days for the United
States that year will be used and the time will be 2
AM. If the start and end dates are provided but the time
is not provided, the time will be 2 AM. The effects of
tzset() are thus to change the values of the external variables timezone, altzone,
daylight, and tzname. The ctime(), localtime(), mktime(), and strftime() functions also update these
external variables as if they had called tzset() at the time specified
by the time_t or struct tm value that they are converting.

Bugs

The zoneinfo timezone data files do not transition past Tue Jan 19
03:14:07 2038 UTC. Therefore for 64-bit applications using zoneinfo timezones, calculations
beyond this date might not use the correct offset from standard time,
and could return incorrect values. This affects the 64-bit version of localtime(), localtime_r(),
ctime(), and ctime_r().

Attributes

The asctime(), ctime(), gmtime(), and localtime() functions are safe to use in
multithread applications because they employ thread-specific data. However, their use is discouraged
because standards do not require them to be thread-safe. The asctime_r() and gmtime_r()
functions are MT-Safe. The ctime_r(), localtime_r(), and tzset() functions are MT-Safe in
multithread applications, as long as no user-defined function directly modifies one of the
following variables: timezone, altzone, daylight, and tzname. These four variables are not
MT-Safe to access. They are modified by the tzset() function in an
MT-Safe manner. The mktime(), localtime_r(), and ctime_r() functions call tzset().

Notes

The return values for asctime(), ctime(), gmtime(), and localtime() point to thread-specific
data whose content is overwritten by each call by the same thread.

Setting the time during the interval of change from timezone to altzone
or vice versa can produce unpredictable results. The system administrator must change
the Julian start and end days annually.

If tzset() has previously evaluated the timezone identified by the value of
the TZ environment variable, tzset() can reuse the previous settings of
the external variables altzone, daylight, timezone, and tzname[] associated with that timezone.

Solaris 2.4 and earlier releases provided definitions of the ctime_r(), localtime_r(), gmtime_r(),
and asctime_r() functions as specified in POSIX.1c Draft 6. The final POSIX.1c
standard changed the interface for ctime_r() and asctime_r(). Support for the Draft
6 interface is provided for compatibility only and might not be supported
in future releases. New applications and libraries should use the standard-conforming interface.

For POSIX.1c-conforming applications, the _POSIX_PTHREAD_SEMANTICS and _REENTRANT flags are automatically turned on
by defining the _POSIX_C_SOURCE flag with a value >= 199506L.

In Solaris 10, gmtime(), gmtime_r(), localtime(), and localtime_r() were updated to return
a null pointer if an error is detected. This change was based
on the SUSv3 specification. See standards(5).